1. Field of the Invention
The present invention relates to a picture data communication system. The system according to the present invention is used, for example, for picture data communication in a television conference system.
2. Description of the Related Arts
In general, in the transmitter side of a television conference picture data communication system, a pixel data sequence is block-divided and supplied to an encoding portion of a differential PCM type in which the encoding for difference representation and subsequent quantization is carried out. In a transmission of picture signals, a great amount of data is transmitted, and thus it is necessary to carry out a further encoding for a bandwidth reduction. Accordingly, the output of the encoding portion is supplied to a bandwidth reducing coding portion, such as a variable length code and run length code encoding portion, in which the assignment of variable length codes for difference representation and quantization and run length code is carried out.
The output of the encoding portion for difference representation and quantization is represented by quantization levels of, for example, 4 to 5 bits. The frequency or probability of occurrence is high, e.g., 90%, for relatively low quantization levels, such as =2, =1, 0, -1, and -2.
In the variable length code encoding portion, a sequence of data is produced in which the codes for a high occurrence frequency are represented by relatively short bits and the codes for a low occurrence frequency are represented by relatively long bits. The encoding by the variable length code makes it possible to reduce the amount of generated data, compared with the encoding by the fixed length code.
Also, in the transmission of picture signals in which the sequence of "0" occurs frequently, it has been acknowledged that the run length coding technique is particularly effective for realizing a bandwidth reduction.
As described above, in the prior art, the manner of encoding pixel data is always fixed regardless of the nature of the input picture data. This fixing of the encoding manner has been found to be disadvantageous for attaining the most suitable picture scanning and data encoding, since the prior art encoding of pixel data deteriorates the efficiency of the transmission of data.